Allicin is an
organosulfur compound
Organosulfur chemistry is the study of the properties and synthesis of organosulfur compounds, which are organic compounds that contain sulfur. They are often associated with foul odors, but many of the sweetest compounds known are organosulfur der ...
obtained from
garlic and
leeks.
When fresh garlic is chopped or crushed, the enzyme
alliinase converts
alliin into allicin, which is responsible for the aroma of fresh garlic. Allicin is unstable and quickly changes into a series of other sulfur-containing compounds such as
diallyl disulfide.
Allicin is an
antifeedant, i.e. the defense mechanism against attacks by pests on the garlic plant.
Allicin is an oily, slightly yellow liquid that gives garlic its distinctive odor. It is a
thioester
In organic chemistry, thioesters are organosulfur compounds with the molecular structure . They are analogous to carboxylate esters () with the sulfur in the thioester replacing oxygen in the carboxylate ester, as implied by the thio- prefix ...
of
sulfenic acid
In chemistry, a sulfenic acid is an organosulfur compound and oxoacid with the general formula . It is the first member of the family of organosulfur oxoacids, which also include sulfinic acids () and sulfonic acids (), respectively. The base mem ...
. It is also known as allyl thiosulfinate.
Its biological activity can be attributed to both its antioxidant activity and its reaction with thiol-containing proteins.
Structure and occurrence
Allicin features the
thiosulfinate functional group, R-S-(O)-S-R. The compound is not present in garlic unless tissue damage occurs,
and is formed by the action of the enzyme
alliinase on
alliin.
Allicin is
chiral
Chirality () is a property of asymmetry important in several branches of science. The word ''chirality'' is derived from the Greek language, Greek (''kheir''), "hand", a familiar chiral object.
An object or a system is ''chiral'' if it is dist ...
but occurs naturally only as a
racemate.
The
racemic form can also be generated by oxidation of
diallyl disulfide:
:(SCH
2CH=CH
2)
2 + 2 RCO
3H + H
2O → 2 CH
2=CHCH
2SOH + 2 RCO
2H
:2 CH
2=CHCH
2SOH → CH
2=CHCH
2S(O)SCH
2CH=CH
2 + H
2O
Alliinase is irreversibly deactivated below pH 3; as such, allicin is generally not produced in the body from the consumption of fresh or powdered garlic. Furthermore, allicin can be unstable, breaking down within 16 hours at 23 °C.
Biosynthesis
The biosynthesis of allicin commences with the conversion of cysteine into
''S''-allyl-L-cysteine. Oxidation of this thioether gives the sulfoxide (
alliin). The enzyme
alliinase, which contains
pyridoxal phosphate (PLP), cleaves alliin, generating allylsulfenic acid (CH
2=CHCH
2SOH), pyruvate, and ammonium ions.
At room temperature, two molecules of allylsulfenic acid condense to form allicin.
[
]
Research
Allicin has been studied for its potential to treat various kinds of multiple drug resistance bacterial infections, as well as viral and fungal infections in vitro
''In vitro'' (meaning ''in glass'', or ''in the glass'') Research, studies are performed with Cell (biology), cells or biological molecules outside their normal biological context. Colloquially called "test-tube experiments", these studies in ...
, but as of 2016, the safety and efficacy of allicin to treat infections in people was unclear.
A Cochrane review found there to be insufficient clinical evidence regarding the effects of allicin in preventing or treating common cold
The common cold, or the cold, is a virus, viral infectious disease of the upper respiratory tract that primarily affects the Respiratory epithelium, respiratory mucosa of the human nose, nose, throat, Paranasal sinuses, sinuses, and larynx. ...
.
History
It was first isolated and studied in the laboratory by Chester J. Cavallito and John Hays Bailey in 1944.[
Allicin was discovered as part of efforts to create ]thiamine
Thiamine, also known as thiamin and vitamin B1, is a vitamin – an Nutrient#Micronutrients, essential micronutrient for humans and animals. It is found in food and commercially synthesized to be a dietary supplement or medication. Phosp ...
derivatives in the 1940s, mainly in Japan. Allicin became a model for medicinal chemistry
Medicinal or pharmaceutical chemistry is a scientific discipline at the intersection of chemistry and pharmacy involved with drug design, designing and developing pharmaceutical medication, drugs. Medicinal chemistry involves the identification, ...
efforts to create other thiamine disulfides. The results included sulbutiamine, fursultiamine (thiamine tetrahydrofurfuryl disulfide) and benfothiamine. These compounds are hydrophobic, easily pass from the intestines to the bloodstream, and are reduced to thiamine by cysteine or glutathione.
See also
* Allyl isothiocyanate, the active piquant chemical in mustard, radishes, horseradish
Horseradish (''Armoracia rusticana'', syn. ''Cochlearia armoracia'') is a perennial plant of the family Brassicaceae (which also includes Mustard plant, mustard, wasabi, broccoli, cabbage, and radish). It is a root vegetable, cultivated and us ...
and wasabi
Wasabi (Japanese language, Japanese: , , or , ) or Japanese horseradish (''Eutrema japonicum'' syn. ''Wasabia japonica'') is a plant of the family Brassicaceae, which also includes horseradish and Mustard plant, mustard in other genus, genera. ...
* ''syn''-Propanethial-''S''-oxide, the lachrymatory chemical found in onion
An onion (''Allium cepa'' , from Latin ), also known as the bulb onion or common onion, is a vegetable that is the most widely cultivated species of the genus '' Allium''. The shallot is a botanical variety of the onion which was classifie ...
s
* List of phytochemicals in food
References
{{Authority control
Thiosulfinates
Anti-inflammatory agents
Antibiotics
Dietary antioxidants
Pungent flavors
Allium
Garlic
Antifungals
Allyl compounds
Transient receptor potential channel modulators